Flow cytometric dot plots of cerebrospinal fluid specimens demonstrating lymphoma populations. Each dot represents a single cell. The units on the x- and y-axes are relative fluorescent intensity. (A) Monoclonal B-cell population. All lymphoma cells in the oval area are λ-positive (y-axis) and κ-negative (x-axis). (B) Surface immunoglobulin-negative or dim B-cell lymphoma. The B cells in the oval area are CD22⁺ (y-axis) and negative for κ on the left and λ on the right (x-axis). (C) Small population of monoclonal B cells in a specimen with low cellularity. The B cells in the oval area are CD22⁺ (y-axis) and κ-positive (left, x-axis) and λ-negative (right, x-axis). (D) CD20^– B-cell lymphoma. The B cells in the oval area are CD20^– (x-axis) and CD22⁺ (y-axis) and monoclonal (not shown). The CD22^– and CD20^– cells are T cells.